Automatization of electro-oculographic examination

ABSTRACT

The invention is especially related to a method for performing an electro-oculographic (EOG) examination of the eyes. Alternating optical signals are produced (34, 36) for the stimulation of the eye movement and those changes in the bioelectric potentials are observed (12), which changes are caused by the eye movements corresponding to said optical signals, for the establishment of sample signals. Noise signals are filtered (22) off by forming a moving average from successive sample signal portions, defining from said average signals the potential leaps or transitions in the biopotential signals, which transitions are caused by said eye movements, and calculating (24) values corresponding to said transitions. Possible distorted values are removed from the set of said values and finally the EOG ratio is defined on the basis of values selected from the remaining set of values. 
     More generally the invention relates to a method for determining, from a sample signal comprising potential leaps and spurious noise, a reference value for a potential leap. From the analogue sample signal a digital signal succession is formed from the successive signal portions of which moving averages are calculated. An approximate potential leap point is obtained from said average signals by windowing at an area where a transition is supposed to exist. 
     The invention further relates to an apparatus for the realization of an EOG examination in accordance with a method disclosed above.

The present invention relates to a method as defined in the introductorypart of claim 1, as well as to an apparatus for the realization of saidmethod for carrying out an electro-oculographic (EOG) examination, saidmethod being used for detecting certain types of damages in the retina.Especially the invention relates to a method for speeding up andfacilitating to the EOG examination by automatizing it. More generallythe invention is directed to a method for determining a reference valuefor a potential leap from a sample signal comprising potential leaps andspurious noise.

The invention further is related to an apparatus as defined in theintroductory part of claim 7, for the realization of the methodaccording to the invention.

Electro-oculography is an electro-physiological examination centrallyrelated to opthalmology, said examination being based on the measurementof slow fluctuations in the electrical potential differences between theretina (- pole) and the cornea (+ pole). The ratio between saidcornea-retinal potentials is called the EOG-ratio. EOG is used for theexamination of certain degeneration diseases in the retina. It mayfurther be used for ascertaining whether a medication used e.g. forcuring rheumatism is damaging the retina. Thus, EOG examination isconsidered to be essential in patient examination, but it is alsoconsidered to be a good examination method in experimental studies andespecially when carrying out retina toxic examinations for medicines.

The potential cannot be directly measured at the eye. This could be doneby arranging an electrode on the cornea, but in this case the eye mustbe anesthetized, which would lead to erroneous results due to theabnormal function of the eye.

This problem is avoided by measuring the EOG ratio indirectly usingelectrodes arranged in the vicinity of the patient's eyes, wherebymutually different biopotentials, which can be measured, are formed bythe movement of the eyes. The biopotential ratio for an eye can then bedefined by calculation from the mutual ratio of said bio potentials.Depending on the test arrangement a varying amount of electrodes arearranged on the skin, and further there are numerous differentpositioning alternatives for them. The movement of the eyes isaccomplished using e.g. blinking light sources. In a widely used testarrangement the patient looks at two light sources arranged at oppositesides of his eyes. The lights blink in opposite phases at a frequency ofe.g. 1 Hz, so that the patient's eyes are in a constant movement backand forth. Also such a test arrangement is known where a fixed light orsome other distinct target is moved in front of the patient's eyes.

The potential always varies when the patient moves his eyes. The wavefrequency of the potential amplitudes thus initiated is identical withthe blinking frequency of the light source.

The size of the potential amplitude is proportional to the EOG ratio,since the potential is zero when the eye looks straight ahead andcorrespondingly differs from zero when the eye looks sideways orupwards.

Since the potential depends on the position of the eye and on thecornea-retinal potential it is impossible to define the realcornea-retinal potential. This fact, however, is unsignificant, sincethe potential ratio achieved as a result of the examination is moreimportant. It is, however, important that during the examination the eyemoves between the same suitably defined points.

The examination itself is usually started in darkness. The potentialdifference signal is recorded for about 10 seconds once in every minuteduring a fifteen minute period. The signal is not measured continuously,because the continuous moving of the eyes is very exhausting, whichwould distort the results. Also, the changes in this type ofcornea-retinal potential are very slow.

First the cornea-retinal potential decreases slowly when the eye adaptsto the darkness. This should happen in about 8 to 9 minutes from thebeginning of the examination. After this dark fifteen-minute period thelight is changed to a very bright one. Then another fifteen-minuteperiod is recorded. The cornea-retinal potential should rise as the eyeadapts to the new lighting. This takes again about 8 to 9 minutes.

After this the EOG ratio is defined taking the highest and lowestcornea-retinal potential values and calculating their ratio. If thisratio is below a defined limit the case usually needs closerinvestigations.

However, manually performing this kind of examination has been a verytedious task. Different apparatus, like the light source, the measuringequipment and the plotter, must be started and stopped a number oftimes. One must know how to select the "good" samples from among theones obtained, and their average values must be calculated, after whicha graphic presentation is made for the average values of eachexamination period, from which presentation the EOG ratio finally can bedefined. It is usual to obtain about 1000 "good" potential amplitudes tobe defined manually, which increases the calculation error possibilityat the average value calculation. In the examination it is furtherpossible to include several special functions which also demand muchattention from the examiner.

For said reasons an automatic performance of the examination sequencehas been suggested, e.g. controlled by a processor or logical circuit.In U.S. Pat. No. 4,474,186 there is suggested an apparatus whichautomatically controls an electro-oculographic examination andautomatically processes the results derived from the examination andpresents these results in a readable form. The arrangement thusdeveloped is, however, rather complicated and the results obtainedtherefrom have not proven to be exact enough. Especially the weakness ofthe amplitudes obtained from the electrodes has caused unexactness,since the relevant signals tend to be covered by random electromagneticand biological background noise, said noise distorting the signalobtained and essentially complicating the achievement of reliableresults. The ability of the electronic error filter system in theapparatus according to the cited document to separate "good" and "poor"samples from each other has also proven insufficient, since it is basedon a low and high pass filtering of the noise signals. However, theelectrodes provides such a weak signal that it is especially difficultto select filters capable of providing a sufficient amount of reliableresults after filtering which still not simultaneously filtering away aconsiderable portion of the signal. Especially due to the high errorprobability in the results given by the apparatus according to the citedpublication no significant improvement in the EOG examination hasactually been achieved therewith.

Further, in the publications DE 3511695 and DE 3511697 complicatedsystems for the automatization of EOG examinations are disclosed. Inthese known systems the filtering of the signals and the poorreliability of the results also constitute a problem, since both thesystems according to said cited publications use low and high passfiltering for the noise reduction.

Further, all the known systems discussed above have been confronted withthe fact that the computer's pattern recognition ability isinsufficient. The computer has not been able to define exact enough EOGvalues from the signal graph obtained, and especially the insufficientpattern recognition ability has increased the asymmetry andnon-continuousness in the graphs.

The object of the present invention is to overcome the disadvantages inthe prior art and provide a quite new solution for performing anautomatic EOG examination exactly and quickly, in a reliable and evenvery simple manner.

Another object of the invention is to provide a functionally simple andeasy-to-use method and apparatus for the performance of an EOGexamination.

Yet another object of the invention is to provide an apparatus andmethod which give the results of an EOG examination in real-time.

Yet another object of the invention is to provide such an apparatus forthe automatic performing of an EOG examination, which apparatus may berealized using easily accessible standard components and a microprocessor.

The invention is based on the idea that the signal obtained from theelectrodes is subjected to a computer aided median filtering, i.e. amoving average value is defined from the signal's subsets, the signalfavorably being digitized prior to the median filtering. The filteredsignal is derivated and, using the peak of the derivative signal and asecond point defined in accordance with the teaching of the presentinvention, the real level of the potential leap is defined byintegration. Using the method according to the present invention thesystem's ability to differentiate between "good" and "poor" signals willbe essentially improved.

In this connection it should be observed that the digitalized signal isnot continuous but represents the peak value for samples taken per timeunit. In a purely mathematical observation such a non-continuous signalcannot have any derivative. Since, however, the difference between twoadjacent samples will behave- like a derivative it will, in thisconnection, be called a derivative, which can be expressed as:

    dx.sub.n =x.sub.n -x.sub.n-1,

where n is an integer

In this connection the term integral is used to express thecorresponding relation between two adjacent samples, which behaves likean integral and can be expressed as: ##EQU1## Further, in thisconnection the term transition will be used to indicate a potentialleap.

More exactly expressed, the method according to the invention is mainlycharacterized by the features disclosed in the characterizing parts ofclaims 1 and 6. The apparatus according to the invention ischaracterized by the features disclosed in the characterizing part ofclaim 7. Other characterizing features are found in the dependentclaims.

According to a preferred embodiment of the invention an EOG examinationof the eyes is performed automatically so that the examiner does not atany stage after the initiation of the examination cycle need tointerfere with the examination procedure, but the system according tothe invention will independently perform the action cycles of theexamination, perform the removal of error signals, analyze the obtainedmeasured results as well as store and print them in a desired form.

Reliable and exact results will be obtained using the method accordingto the invention, where signals obtained from electrodes arranged on theskin will be digitalized and median filtered for the elimination oferroneous signals. The signal obtained is derivated whereby anessentially symmetric triangular form is achieved for the obtainedsignal pattern.

In the next step the minimum and maximum of the essentially triangularderivative signal is localized. In this respect forming of a window maybe used, the window being applied around that point where a maximum isbelieved to occur. The length of the window is preferably defined as thelength of one phase of the light source flash. The centre of the windowwill not be exactly where it theoretically should be, since due to thereaction time a person will move his eyes with a slight lag with respectto the light source and thus also the window will be located slightlylater than the light source cycle.

At the setting of the windows it should further be investigated whethera minimum or a maximum of the signal will occur first. This is done byassuming a maximum to come first and setting the windows accordingly.Thereafter the signal values are multiplied with 1 in maximum windowsand with -1 in minimum windows. If the average of the values thuscalculated is positive the presumption was correct and if the averagevalue is negative the first window was located at a minimum.

Now the exact positions of the peaks are known. Next the position of theedges of the triangular peak area should be found. The remaining noisein the signal makes it impossible to define the position of the edgesusing the signal zero points. Further, the last edge of a peak isnormally further away than the first edge.

For this procedure the form of the peak is utilized. After the medianfiltering the peak should have straight edges. Finding the position ofthe edges and the peak end is started at the maximum and the samples aresearched one after the other, moving forwards away from the maximumuntil the average of all the samples between the present position andthe maximum is half the maximum. Using this position point and themaximum point the end point of the peak is obtained for the calculationof the area.

Finally the amplitude size is simply obtained by integrating over thepeak area of the curve, whereby the real amplitude height is obtained.

The calculation is preferably performed using a computer and it canpreferably be made faster by combining the median filter and derivationstages.

The method according to the invention will give several remarkableadvantages. Not only will the analysis of the signals obtained from theelectrodes be faster with respect to prior art, but the obtained resultswill also be more exact and reliable. The apparatus needed forperforming the examination and the method according to the invention iseasy to realize and favorable regarding the costs, since all thecomponents needed are of a type which can be commercially obtained.

Below an embodiment of the invention will be described with reference tothe accompanying drawings, where:

FIGS. 1a and 1b schematically show the electrical circuit formed by theeyes and they show the potential level when the eye looks straight aheadand to the side,

FIG. 2 is a schematic disclosure of the main idea of the invention,

FIG. 3 is a schematic disclosure of an apparatus according to onepreferred embodiment,

FIGS. 4a and 4b show a measured signal,

FIG. 5 shows the same signal as derivated but without median filtering,

FIGS. 6A-6D' shows measured results obtained using different filterlengths 1, in FIGS. 6A and 6A' 1=5; in FIGS. 6B and 6B' 1-19; in FIGS.6C and 6C' 1=20; and in FIGS. 6D and 6D' 1=40,

FIG. 7 shows sizes of potential leaps calculated from measured signalvalues, and

FIG. 8 shows EOG values obtained in one examination.

More precisely, FIG. 1a discloses the electrical circuit formed by ahuman eye, and its potential when said eye looks straight ahead.Favorably the nose base and the temples are used as measuring points forthe biopotential, i.e. as attachment points of the measuring electrodes12, but other locations may be used too. Said electrodes 12 areconnected to an amplifier 14, which can be a conventional EKG-apparatusor the like. As can be seen from the figure, the potential of thecircuit is essentially zero when the eye is looking as indicated in thefigure.

In FIG. 1b the eye is shown turned to the left, and it can be observed,that the temple electrode potential now is higher than that of theelectrode 12 at the nose base.

During the examination procedure it is important that the eye movesbetween the same two points. Further a too excessive angle between lightsources will cause the patient's eyes to be rapidly wearied and then thesignal will loose its sharp form. On the other hand, a too narrow anglewill give too low potential leap values from which it is impossible todefine the EOG ratio. As a suitable angle would be about 15 from thecenter line to both sides of eyes, as indicated above.

FIG. 2 schematically shows the basic idea of the present invention inone of its simplest embodiments. According to the figure the signalobtained from the electrodes 12 is amplified in an EKG-amplifier 14,filtered in a median filtering means 22, which is functionally connectedto said amplifier, the correct signal transition is obtained by acalculation means 24, which is functionally connected to said medianfilter, and said signal is favorably stored and displayed in storage anddisplay means 26. Said median filtering means 22 and calculation means24 may also favorably be combined, as indicated by a line of dots anddashes, whereupon they will simultaneously handle the same signal.Further, the operation of the blinking lights 36, 38 and the generallights 34 are controlled by incentive control means 30. All thesequences performed by these means, except for the amplifying phaseperformed by the amplifier 14, can preferably be performedprogrammatically using a microcomputer.

FIG. 3 schematically shows a favourable apparatus embodying theinvention, said apparatus being generally indicated by the reference 10.Said apparatus comprises electrodes 12, which are functionally connectedto EKG-amplifiers 14. Usually one such amplifier is needed for each eye.Said EKG-amplifiers 14 are functionally connected to a selector device16 which preferably is connected to an A/D converter device 18. Saidapparatus 10 further comprises a computer 20, which is functionallyconnected to said converter 18 and comprises said median filtering means22, said transition calculator means 24 as well as said storage anddisplay means 26. Said computer 20 further comprises said incentivecontrol means 28, which means are functionally connected to the actualincentive controller 30. Said controller 30 controls the function ofsaid general light 34 which is functionally connected to said blinkingincentive lights 36, 38 and a relay 32.

Said amplifiers 14 must be completely isolated, i.e. none of the threeelectrodes may have a fixed potential. This is very important withrespect to the patient security.

Another important feature is that said amplifier 14 comprises threeelectrodes 12. Two of said electrodes are differential electrodes,between which the potential difference is measured. The third electrodeis an active zero electrode which feeds the measured signal back to thepatient, which feature will decrease the measured noise level. Thisespecially attenuates the strong 50 Hz signal.

A test arrangement set up according to FIG. 3 comprised two amplifiers,both being Kone 521 EKG amplifiers. in order to reduce the number ofelectrodes needed the active zero electrodes of each amplifier wereinterconnected. This is possible since said electrodes are completelyisolated. Said test arrangement further comprised a notch filter tunedfor 50 Hz.

The amplification factor for the amplifiers used is 2000. This is quitesuitable because a typical potential difference in an EOG examination isabout 1 mV. Thus the amplifier's output voltage is between -2V and +2V.

The interface card for the computer included in the test apparatuscomprises several functions when an EOG value is measured. It stores allmeasured results, controls said light sources and acts as voltage supplyfor said EKG amplifiers.

The used computer interface card had the following properties:

a two channel A/D converter having high impedance inputs

a power source (60 V_(pp) 20 KHz) for the isolation transformers in theamplifier

a TTL level signal output for controlling the light source and 2 outputsfor controlling the LEDs

a direct connection to an IBM PC or a PC/AT ISA data bus.

Since this kind of ready made cards could not be obtained said card wasconstructed and composed according to the above disclosed principles onan IBM card having a ready connection to the ISA data bus and an addressencoding logic. For the person skilled in the art it is evident that theprocessing arrangement according to the invention as such can beperformed in several different manners.

The next task comprises the definition of the size of the potentialleaps from the EKG signal. For human beings this is not any especiallydifficult task, since humans have a rather developed pattern recognitionability and thus a slight noise in the measured signal does notsignificantly disturb the recognition process. Unfortunately, a computercannot perform such a pattern recognition, and thus mathematical rulesbased on statistical methods must be deduced for this purpose.

As already is mentioned, the leaps are quite evident for a human being.Unfortunately they cannot be found at standard locations, since eachpatient will move his eyes at a slightly different speed. Thus, eachstep must be found independently.

FIG. 4b discloses the signal after derivation, which signal in FIG. 4ais disclosed without a median filtering. The derivation is performed inorder to define the points where the patient moves his eyes. As evidentfrom FIG. 4b a random noise will produce a derivative which isundulating to quite a high extent and which has very little regularity,except for a rather clear initial signal. From this curve it isdifficult even for the human pattern recognition ability to findsimilarities with the actual signal curve. Thus, it is important toreduce the disturbing amount of noise.

In FIG. 5 one derivated signal peak is disclosed. As is evident fromthis figure the median filtering broadens the peak producing a clearlytriangular form. The processing of said peak starts with the defining ofthe maximum of said peak. Here the problem is to find a local maximumx_(p). The task is not difficult if the signal is as clear and clean asthe signal shown in FIG. 5, but in practice the patient's eyes sometimes"get lost", i.e. they look somewhere else and not at the light source,and-this produces small local maxima. These error maxima are excludedfrom the real maxima in a manner to be disclosed hereafter.

It is known that the real peaks come in a regular order, i.e. a maximummust follow a minimum and vice versa. Further, it is clear that even ifthe exact eye movement interval is unknown the number of maxima andminima during the light blinking is in the range from n-1 to n+1. Inother words it is known where the peaks should be located, i.e. theyshould be at the same location where the light source flashes. It mustof course be understood that the peaks cannot be exactly at thatlocation since due to the human reaction time the patients eyes reactslightly behind the light source. This problem is removed by assuming awindow around the location of the peak, the length 1 of said windowbeing identical to the illumination time for one light source. Further,it is preferable to locate the center of the window slightly behind thetheoretical maximum since it is much more probable that the patient willmove his eyes slightly after the light source than before it.

Here it should further be analyzed whether the first window comprises amaximum or a minimum. The signal phase can be investigated by modulatingit with a "window signal". This is accomplished by first assuming thecoming signal to be a maximum, and setting the windows accordingly.Thereafter the signal sample peak values are multiplied with the value 1in the maximum windows and with the value -1 in the minimum windows. Ifthe average of the values thus calculated is positive the assumption ofthe first coming maximum was correct, and if the average becomesnegative the first window was set at the location of a minimum.

Thus, the exact positions of the peaks and their maxima x_(p) areobtained. The next step comprises the exact position of the edges of thetriangular area of the peak. The rest noise in the signal makes thedefinition of the signal's edges using the zero points impossible.Moreover, the last edge of the peak will usually be located farther fromthe zero point than the first edge.

Here the form of the peak is used as a help. After the median filteringthe peak should have straight edges. In FIG. 5 the edges of such atheoretical ideal peak is indicated in phantom line between said maximumx_(p) and zero points x_(e). From said figure it is evident that if theaverage of a local maximum x_(p) and a point x_(e) is half of themaximum value said point x_(e) is a zero point for said peak. Inpractice, finding the edges and zero points starts from a maximum valueand the samples are analyzed one after the other proceeding away fromsaid maximum, until the average of the investigated samples is half themaximum value. A straight line through this point and said maximum pointdefines the zero point x_(e) for the peak, which point is needed in thecalculations. It should be observed that even if it usually is favorableto calculate said average point for only one of the triangle's sides andthereafter assume the triangle to be an isosceles due to signalsymmetry, also the average value for the first edge can be calculatedand then the triangle need not necessarily be an isosceles.

Using the maximum points x_(p) and the zero points x_(e) obtained inaccordance with the above, the area of the triangle peak can easily becalculated. Since the peak represents the derivative for the measuredsignal the potential leap height can be obtained by integrating over thepeak area. This can preferably be done by summing the signal valuesbetween said points x_(e) and then the sum obtained corresponds to theheight of one leap. This equation can be expressed as: ##EQU2## wherea=the leap size

x=the derivative signal

s and e are the edges of the peak.

FIG. 6 shows some measured signals (FIGS. 6A, 6B, 6C and 6D) andderivatives (FIGS. 6A', 6B', 6C', and 6D') obtained therefrom usingdifferent median filtering lengths 1. As can be seen the obtainedderivatives have a very symmetrical form. There is also considerablyless rounding of the edges than can be achieved by conventional low passfiltering. It is easier to observe peaks having a regular form andregular edges than it would be in case of a low pass filtered signal. Inthe example case a median filter length of 1=5 was chosen since it willeliminate most of the noise but hardly distorts the form of the signalat all.

Now there is obtained the size of about 2*30*10 peaks, which will beformed for two eyes during 30 minutes from values taken 10 times aminute. Now a representative for each minute should be calculated fromthese ten values. FIG. 7 shows as an example the sizes of potentialleaps calculated from values for one minute as measured during an actualtest series. It is probable that the first value is not reliable. If themeasuring series comprise one or more clearly erroneous results likethat above, one cannot simply take an average value of the measuredseries and define it to represent the whole one minute measuring series.

Such error values cannot be eliminated by set threshold values, sincethe real values vary a lot. One favorable method that has proven to besufficiently exact is to search for n samples which are as close to eachother as possible. The integer n should be sufficiently small in orderto eliminate erroneous samples. In tests a value for n corresponding tohalf the amount of the measured values has proven advantageous. If thenumber of erroneous values is more than half the amount it is ratherimpossible with any method to find the real values.

The n specimen of values to be obtained are the points which have thesmallest standard deviation and they can be found as follows:

First the average value of the whole set of points is calculated.Thereafter the most remote-point is removed. After this the averagevalue is calculated for the remaining set of points and again theremotest point is removed. This procedure is repeated until there are npoints left. Hereafter the average of this set of points represents thewhole set of points.

In this manner the number of values needed for the calculation of thecoronea-retinal potential is reduced to 2*30 values. In the next stepthe EOG ratio is calculated from these values. FIG. 8 shows a curverepresenting such values in a test examination.

Often it is favorable to define the EOG ratio by looking for the curveminimum in the dark interval between 5 and 12 minutes. A minimum foundshould be checked with respect to its neighbor values in order toascertain its reliability. A maximum value is obtained in acorresponding manner but from the light test period area. From themaxima and minima obtained the EOG ratio is finally calculated, printedand stored at some preferred means.

Until now an automatic determining of clearly expressable quantitiesfrom such signals has proven to be very difficult and unexact. However,the method according to the invention brings about an essentialimprovement. According to the invention there is now utilized a medianfiltering, where the signals firstly are suitably digitized in order toexploit all the benefits of the median filtering. Median filtering isnamely a very effective way to reduce such random deficiencies whichdepend on the environment and even on the examined patient's brainfunctions, which deficiencies in the prior art have constituted a realproblem.

The median filtering or calculating the moving average value of thesignals is performed by defining a new value for each new pointutilizing the average of its neighboring points. This filtering methodreduces the noise very efficiently, since the average value for therandom noise is zero over the infinite interval. Of course, the intervalused in the practical solutions is not infinite, but the filter stillattenuates the noise very well. The noise amplitude will roughly beattenuated by a factor 1 (i.e. the length of the filter).

The formula of the median filter is ##EQU3##

All filters that eliminate noise also destroy some significantinformation. Also this filter rounds the signal edges, but the length ofthis rounding is rather short since only the points in near proximityaffect the filtered point. This is because the points further than 1from the point to be filtered cannot have any effect on the result. Inthis respect a median filter decisively differs from e.g. a low passfilter where all preceding points affect the filtered point.

In order to obtain a signal having clear peaks the signal now must beprocessed. In a computer this is preferably done in real time so thatthe number of operations is minimized, and thus the effectsimultaneously is maximized. An effective method for reducing the numberof operations is often to perform them simultaneously. In this case thisis favorably done by combining the median filtering and thedifferentiating. In the following formulae x is the original signal, yis the median filtered signal and dy is the derivative of the medianfiltered signal: ##EQU4##

When these formulae are combined the following formula is obtained:##EQU5## which for most of the practical embodiments can be reduced to:

    z.sub.1 =x.sub.1+l -x.sub.i-l-1                            (D)

The next step in the signal processing is to find the maxima and minimaof the derivative signal, and the first problem then is to find localmaxima. The task would not be difficult if the signal were completelyclean but in the signal in practice there usually are small falsemaxima, due to the fact that the patient's eye sometimes "gets lost",i.e. it does not actually look directly at the light source 36, 38.However, the correct maxima should be found, and this is performed inthe manner described above. When the maxima are found according to theabove the edges of the peaks are defined and finally the amplitude sizeis formed using integration, as above, has been described in greaterdetail. As the signal in question is a digitized one this can normallybe easily performed by summing all signal values between the beginningand end points of a peak value.

Thereafter the EOG ratio must still be calculated on the basis of a setof values consisting of several peak values. In this set there probablystill will exist also such values which for some reason do not representtypical values but rather should be considered as errors. For thisreason those values which are considered to best correspond to thedes-red properties are separated for the calculation. According to asimple solution this elimination is performed using always smallersubsets so that the average of the remaining set is calculated and thefarthest value is removed therefrom until suitably about the half of theoriginal values are left. From this subset of values the EOG ratio isnow calculated in a manner known per se.

Above a preferred embodiment of the invention has been disclosed, butfor the person skilled in the art it is clear that the invention can bevaried and adapted in many other ways within the scope of the appendedclaims.

In addition to the above the enclosed program utilized in the exampleembodiment is included within this presentation. ##SPC1##

I claim:
 1. A method for performing an electro-oculographic (EOG)examination, wherein alternating optical signals are produced (34, 36)for the stimulation of eye movements and, for the establishment ofsample signals, those changes in the bioelectric potential signals aredetected (12), which changes are caused by the eye movementscorresponding to said optical signals, characterized in filtering (22)noise signals off by forming a moving average from successive samplesignal portions, defining from said average signals potential leaps ortransitions in the biopotential signals, which transitions are caused bysaid eye movements, and calculating (24) values corresponding to saidtransitions, removing possible distorted values from among said valuesand finally defining the EOG ratio on the basis of values selected fromthe remaining set of values.
 2. A method as defined in claim 1,characterized in converting (18) the bioelectric potential signals tonumeric type digital signals prior to the calculation of said movingaverage.
 3. A method as defined in claim 2, characterized in definingfrom the average signals the local positions of extreme values for therate of change, preferably by forming windows at the calculated valuescorresponding to said rate of change in that point which respectivelycorresponds to said alternation of said optical signals, the windowlength then preferably corresponding to the time during which eachalternating optical signal is on, and defining the quality of thedetected extreme value of the rate of change.
 4. A method as defined inclaim 2, characterized in defining the absolute sizes of saidtransitions by determining in an area corresponding to said transitionthe highest individual value (max) for the signal rate of change in apoint (x_(p)) and calculating advancing therefrom averages for the rateof change for successive values smaller than said highest individualvalue up to that point (x_(e)) where said average corresponds to halfthe said highest individual value, at which point (x_(e)) one of thetheoretical zero points is located, the other zero point being definedat a corresponding distance from the location (x_(p)) of said highestvalue on the opposite side therefrom, and defining the absolute size ofsaid transition as the area of a thus formed triangle (x_(e), max,x_(e)), suitably as the sum of all individual values located betweensaid defined zero points (x_(e), x_(e)).
 5. A method as defined in anyof claim 2, characterized in choosing, for the following calculations,from the set of the absolute values for several transitions the mostexact values, suitably about half of all values, by definingcontinuously smaller partial sets of values, where for each partial setan average is calculated and the value which is farthest away from saidaverage is left out, until the desired amount of values is obtained orthe variation is small enough.
 6. A method for determining, from asample signal comprising potential leaps and spurious noise, a referencevalue for a potential leap, characterized in forming from an analoguesample signal a digital signal succession, from the successive signalportions of which moving averages are calculated, obtaining anapproximate potential leap point from said average signals by windowingat an area where a transition is supposed to exist, obtaining in thearea of said potential leap the highest individual value for the rate ofchange, calculating at least one of the theoretical zero points for thechange in the potential leap by forming, starting from said highestindividual value, the averages of successive values smaller than saidhighest individual value, a zero point then being defined in the pointwhere, respectively, the calculated average is half of said highestvalue, and defining a reference value for said potential leaps as thesum of all individual values of the partial set of signals between saidzero points.
 7. An apparatus for the realization of an EOG examinationin accordance with a method disclosed in claim 1, said apparatuscomprising means (34, 36) for stimulating-the movement of the eyes in asynchronized manner, and means (10) for calculating the EOG rate for theeyes on the basis of detected differences in the biopotential,characterized in that said apparatus comprises means (12) for detectingthe differences in biopotential, said differences being caused by themovement of the eyes, means (22) for filtering the top values of signalsformed by said biopotential using median filtering, i.e. means forcalculating the moving average, as well as means (24) for determiningthe transitions or potential leaps for said signal top values.
 8. Anapparatus as defined in claim 7, characterized in that said apparatuscomprises means (14) for detecting and amplifying the observed signals,preferably one or more EKG devices or the like, which preferably aresuch that each comprises three sampler electrodes (12) where saidsampler electrodes (12) preferably are so isolated that they have nofixed potential to any potential level.
 9. An apparatus as defined inclaim 7, characterized in that said apparatus further comprises means(18) for digitizing said signals, said means (18) being connected, withrespect to the advancement direction of said signal, before said means(22) arranged for median filtering.
 10. An apparatus as defined in anyof claim 7, characterized in that said means (22) for accomplishing themedian filtering and said means (24) for determining said transitionsfor said top values comprise mutually integrated means, preferably acomputer (20) and soft-ware attached thereto.
 11. An apparatus asdefined in claim 7, characterized in that said apparatus comprises means(28, 30, 32) for the controlling of means (34, 36, 38) arranged forstimulating the eye's movements, said controlling means (28, 30, 32)functionally being preferably at least partially integral with saidmeans (24) for determining said transition, preferably in connectionwith said computer (20).